Thermoplastic resin films, particularly biaxially stretched polyester films, are widely used in a number of applications such as magnetic recording materials and wrapping materials because of their excellent mechanical properties, electrical properties, dimensional stability, transparency and chemical resistance.
Especially in recent years, polyester films are used in display-component applications such as touch panels, liquid display panels (LCD), plasma display panels (PDP) and organic electroluminescence (organic EL), as well as in various types of films for optical applications.
Particularly in such applications, a hard coat film composed of a polyester film on which a hard coat layer is laminated is used. Further, in order to improve the adhesive property between the polyester film as a substrate and the hard coat layer, a coating layer having an adhesive property is often disposed as an inner layer.
Hard coat films are required to have an adhesive property to a substrate at an ordinary temperature and also under high temperature and high humidity conditions, transparency, abrasion-resistance, and antifouling property. In addition, since hard coat films are often used on the surfaces of displays and the like, they are required to have visibility and design characteristics. In cases where a hard coat layer is laminated on a polyester film as a substrate, and when there is a difference in the refractive indices of the hard coat layer and the substrate polyester film, reduction of interference pattern is needed, because the occurrence of the interference pattern due to interface reflection leads to deterioration of visibility. As a result, after forming a refractive index controlling layer (AR layer, index-matching layer) or an antifouling layer on the hard coat layer, the visibility of the image display device and the like may be deteriorated, or the high-grade feeling may be compromised.
With growing demand for display devices having a larger size, higher definition and higher grade, especially in recent years, the required levels for suppression of interference pattern, particularly under a fluorescent lamp, transparency, and adhesive property between layers are increasing.
On the other hand, in cases where a refractive index controlling layer composed of a high refractive index layer/low refractive index layer is laminated on the surface of the hard coat layer, use of a hard coat layer having a higher refractive index enables to eliminate the necessity of having the high refractive index layer in the refractive index controlling layer. As a result, a large reduction in cost during the film production process can be achieved without compromising its functions. This technique is drawing attention, reflecting a strong demand for cost reduction in recent years.